A few different options for you. YOu can go with a smaller light that are also used on pistols such as the Streamlight TLR-1. they make a kit that has a rifle setup with pressure pad. Nice light that can also be used on your pistols with rail as well as your rifle.
If you want something a little more robust there are the Surefire weaponlight systems. This is absolutely top notch and would work very well for you, tarde off is they are more pricey. Nice thing about some of them is that they can easily be used for a handheld light as well as mounted.
Both mentioned have mounts bult in or included. hope this helps!